As a quickie, let me clarify this little question. If you wish to write about something that is comparatively more dry, you write drier. If you mean a machine or other device that makes something drier, you want the word dryer. A dryer makes things drier. Personally, I avoid the whole things by using "more dry" but technically the comparative form exists. Now excuse me while I makes the driest martini ever while drying my clothes.